1. Field of the Invention
The present invention relates to spherical semiconductor devices, and more specifically to the packaging and surface mounting of spherical semiconductor integrated circuit devices.
2. Description of Related Art
Conventional integrated circuits (ICs) are fabricated on flat silicon wafers. To begin the fabrication process, silicon is melted to form a large single crystal. The crystal is then cut, polished to a smooth finish, and heated in order to form a silicon wafer, which typically is three to twelve inches in diameter. Next, actual integrated circuits are fabricated on the wafer through a design-specific set of processing steps (such as mask patterning, implantation, deposition, diffusion, and etching). At this point, the ICs on the wafer are tested a first time to determine if each is operational. Then, the wafer is diced to separate the individual ICs, and the operational ICs are packaged in a plastic or ceramic molding to form a "chip." The packaged ICs are then tested once again to insure that the completed device is functioning properly.
In such a wafer-based fabrication process, the number of ICs that are produced on each wafer depends on the size of the wafer and the size of the IC being fabricated. Recently, the wafer diameter has been increased in order to increase the manufacturing productivity and decrease the cost per IC device. However, each move to a larger wafer necessitates the purchase of expensive new manufacturing equipment and adds process complexity that can increase the defect rate. Further, when a matrix of individual ICs is formed on a wafer, the ICs must be spaced relatively far apart in order to facilitate their separation in the mechanical dicing process. Due to factors such as the required spacing and the defect rate, the aggregate area of the silicon wafer that yields functioning IC devices can be as low as 10%.
In order to overcome such inherent drawbacks in flat wafer-based IC fabrication, it has been proposed to form an IC device on a silicon sphere. According to this manufacturing process, single crystal silicon spheres (e.g., of one millimeter in diameter) are formed, and each sphere is processed so as to create circuit elements on its outer surface. Because such a one millimeter sphere has a surface area of only 3.14 mm.sup.2, large VLSI circuits cannot be formed on a single sphere. However, larger circuits can be formed by combining multiple spheres in to a single device. In this respect, each sphere can be designed as a portion of a system (e.g., a RAM, ROM, logic circuit, or I/O circuit), and the spherical devices can be connected together to form a complete VLSI circuit. Additionally, the individual spheres can be designed to be "standardized" circuit portions to further simply the device manufacturing process.
The manufacturing of ICs as silicon spheres offers many advantages over conventional flat wafer rectangular ICs. For example, the processing of individual spheres significantly cuts the cycle time needed to form a final device and allows for single devices to be processed (as opposed to batch processing of devices on a wafer). Further, such spherical IC device manufacturing processes can greatly decrease the overall IC device manufacturing cost by eliminating the need for large dedicated clean rooms, by allowing over 90% of the required silicon material to end up in functioning devices, and by eliminating the need to purchase new manufacturing equipment each time technological advances necessitate larger circuit devices. However, while there has been success in fabricating circuit elements on silicon spheres, packaging and mounting schemes are needed in order to allow spherical IC devices to be incorporated into complete systems such as computers and mobile phones.